Big Timing Question

[Dave M]

I couldn't resist thinking too hard before I dropped the M50 back in its home.

In order to generate question for the board :) I removed the one-way valve in a compression tester and jury-rigged it to an air compressor. Voila, very crude leak down tester.

So, I set it to compression TDC, shoot 80 PSI of air into the cylinder and everything seals reasonably well. Then I set it to exhaust TDC and notice that not only are the exhaust valves closed, but the intake valves are slightly open. I've never been this deep before, so excuse me if its a silly question, but, is this thing timed correctly?

Going back out to the garage to get drunk before the bar,

Dave M

[Dave M]

Iy appears I've answered my own question. This may be interesting for those who want to know. Some info on cam timing from the world wide web:


the piston during a critical point in the cam timing knows as the "overlap period". The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short During Overlap both valves are open as the piston rises.
period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve.


Early Intake Valve Opening
In practice, there are many advantages to opening the intake valve early and closing it late. By initiating the opening the intake valve early, the intake valve has time to get to a lift where appreciable flow will begin. On a well-designed cylinder head teamed with a free-flowing exhaust, the pressure in the cylinder when the valve is opened early may be lower than atmospheric, so the intake charge actually gets sucked in (in practice, the exhaust valve is still open when the intake valve begins to open). The benefits of early intake valve opening are very rpm dependent. At low engine speeds, extremely early intake valve opening may cause exhaust gases to be sucked into the intake manifold causing erratic idle and other problems. At higher engine speeds, this same amount of early intake valve opening will have no adverse effects since the intake manifold is not operating under a vacuum condition.


The intake valve starts to open before the piston reaches TDC on the exhaust stroke of the previous cycle. In this way the valve will be fully open by the time the piston moves over TDC and down on the intake stroke. The incoming charge of fuel and air builds up a form of kinetic energy which causes the mixture to continue flowing into the cylinder after the piston has reached BDC and starts up on the compression stroke.

[632 Regal]

I didnt read through all that stuff but my guess is the bottom of the stroke is where the exhaust valves are closing and the intake is closed, another half turn and your at TDC where both sets ov valves should be closed and you can do a leak down :D

hope I co-insided with what you copied and pasted